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SubjectKASAN: use-after-free Read in pppol2tp_connect (3)

syzbot hit the following crash on upstream commit
bcfc1f4554662d8f2429ac8bd96064a59c149754 (Sat Mar 24 16:50:12 2018 +0000)
Merge tag 'pinctrl-v4.16-3' of
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So far this crash happened 5 times on net-next, upstream.
C reproducer:
syzkaller reproducer:
Raw console output:
Kernel config:
compiler: gcc (GCC) 7.1.1 20170620

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l2tp_core: tunl 2: sockfd_lookup(fd=9) returned -9
l2tp_core: tunl 2: sockfd_lookup(fd=8) returned -9
l2tp_core: tunl 2: sockfd_lookup(fd=9) returned -9
BUG: KASAN: use-after-free in __read_once_size include/linux/compiler.h:188
BUG: KASAN: use-after-free in sk_dst_get include/net/sock.h:1767 [inline]
BUG: KASAN: use-after-free in pppol2tp_session_init net/l2tp/l2tp_ppp.c:587
BUG: KASAN: use-after-free in pppol2tp_connect+0x1a98/0x1dd0
Read of size 8 at addr ffff8801d18242a8 by task syzkaller117300/8372

CPU: 1 PID: 8372 Comm: syzkaller117300 Not tainted 4.16.0-rc6+ #365
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:17 [inline]
dump_stack+0x194/0x24d lib/dump_stack.c:53
print_address_description+0x73/0x250 mm/kasan/report.c:256
kasan_report_error mm/kasan/report.c:354 [inline]
kasan_report+0x23c/0x360 mm/kasan/report.c:412
__asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:433
__read_once_size include/linux/compiler.h:188 [inline]
sk_dst_get include/net/sock.h:1767 [inline]
pppol2tp_session_init net/l2tp/l2tp_ppp.c:587 [inline]
pppol2tp_connect+0x1a98/0x1dd0 net/l2tp/l2tp_ppp.c:747
SYSC_connect+0x213/0x4a0 net/socket.c:1639
SyS_connect+0x24/0x30 net/socket.c:1620
do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
RIP: 0033:0x449f39
RSP: 002b:00007fa9d92efce8 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 0000000000700024 RCX: 0000000000449f39
RDX: 000000000000002e RSI: 0000000020e92000 RDI: 0000000000000009
RBP: 0000000000700020 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 000000000080efcf R14: 00007fa9d92f09c0 R15: 0000000000000009

Allocated by task 8381:
save_stack+0x43/0xd0 mm/kasan/kasan.c:447
set_track mm/kasan/kasan.c:459 [inline]
kasan_kmalloc+0xad/0xe0 mm/kasan/kasan.c:552
kasan_slab_alloc+0x12/0x20 mm/kasan/kasan.c:489
kmem_cache_alloc+0x12e/0x760 mm/slab.c:3541
sk_prot_alloc+0x65/0x2a0 net/core/sock.c:1465
sk_alloc+0x105/0x1440 net/core/sock.c:1525
inet_create+0x47c/0xf50 net/ipv4/af_inet.c:320
__sock_create+0x4d4/0x850 net/socket.c:1285
sock_create net/socket.c:1325 [inline]
SYSC_socket net/socket.c:1355 [inline]
SyS_socket+0xeb/0x1d0 net/socket.c:1335
do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287

Freed by task 21:
save_stack+0x43/0xd0 mm/kasan/kasan.c:447
set_track mm/kasan/kasan.c:459 [inline]
__kasan_slab_free+0x11a/0x170 mm/kasan/kasan.c:520
kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:527
__cache_free mm/slab.c:3485 [inline]
kmem_cache_free+0x83/0x2a0 mm/slab.c:3743
sk_prot_free net/core/sock.c:1506 [inline]
__sk_destruct+0x628/0x920 net/core/sock.c:1590
sk_destruct+0x47/0x80 net/core/sock.c:1598
__sk_free+0xf1/0x2b0 net/core/sock.c:1609
sk_free+0x2a/0x40 net/core/sock.c:1620
sock_put include/net/sock.h:1659 [inline]
l2tp_tunnel_free net/l2tp/l2tp_core.c:160 [inline]
l2tp_tunnel_dec_refcount net/l2tp/l2tp_core.h:264 [inline]
l2tp_tunnel_del_work+0x474/0x6a0 net/l2tp/l2tp_core.c:1307
process_one_work+0xc47/0x1bb0 kernel/workqueue.c:2113
worker_thread+0x223/0x1990 kernel/workqueue.c:2247
kthread+0x33c/0x400 kernel/kthread.c:238
ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:406

The buggy address belongs to the object at ffff8801d1824080
which belongs to the cache UDP of size 1472
The buggy address is located 552 bytes inside of
1472-byte region [ffff8801d1824080, ffff8801d1824640)
The buggy address belongs to the page:
page:ffffea0007460900 count:1 mapcount:0 mapping:ffff8801d1824080 index:0x0
compound_mapcount: 0
flags: 0x2fffc0000008100(slab|head)
raw: 02fffc0000008100 ffff8801d1824080 0000000000000000 0000000100000005
raw: ffffea0006b0fda0 ffffea0006a53820 ffff8801d6f36640 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff8801d1824180: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8801d1824200: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ffff8801d1824280: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8801d1824300: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8801d1824380: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

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